Yes meditation is an aspect of yoga. That's an interesting question you ask, and there is not a single, easy answer. There are countless forms of meditation! I believe anything done with conscious intention can be meditative - walking can be a meditation. Washing the dishes can be a meditation.

I consider asana practice (asanas are the physical postures of yoga) to be a moving meditation. One of my favorite yoga quotes says, "The pose starts the moment you want to leave it." So that could be an answer - meditation (that is, focused concentration) starts when things get uncomfortable.

I'm going to assume you really mean how long does it take to relax or enter a higher state of consciousness? Which is what many people seek in meditation. Again, that's not so easy to answer. I know people who have trained themselves to be able to enter those states with a single breath. I know others who struggle every time they "meditate" to calm their mind and let go of the constant judgment of wondering if they are doing it "right", and those people never really unlock the infinite potential of meditation.

I will wrap up this comment by saying that when I started meditating, I didn't really know what to expect, and I had doubts as to if it would work at all. BUT I committed to meditating for just 5 minutes a day, and I had to use a timer for it, but I felt 10 times better after doing that. So I kept doing it, and I kept feeling better, and that was enough reason for me to stick to it even if I didn't have profound mystical experiences every time I sat on my cushion.